Mega Drop-Down Navigation Menus Work Well
http://www.useit.com/alertbox/mega-dropdown-menus.html

Nielsen, Jakob
Alertbox
2009

Abstract:

Given that regular drop-down menus are rife with usability problems, it takes a lot for me to recommend a new form of drop-down. But, as our testing videos show, mega drop-downs overcome the downsides of regular drop-downs. Thus, I can recommend one while warning against the other.